1. Field of the Invention
This invention relates to a laser beam scanning system. More particularly, the invention relates to such a laser beam scanning system which is available for an image writing head for use with image-forming apparatuses like an electrophotographic copying machine, a laser printer, or a facsimile, or the like.
2. Description of Related Art
Generally, any conventional laser beam scanning optical system built in an image-forming apparatus like a laser printer or a facsimile causes a deflecting unit (a polygon mirror) to deflect and scan a laser beam emitted from a laser light source (a laser diode) on a plane at equal angular velocity, and then corrects image-scanning velocity by applying an f.theta. lens or an f.theta. mirror before forming an image on a scanning line (a photosensitive object). Next, in order to detect image-writing starting point per scanning process, the scanning starting part of the laser beam deflected by the deflecting unit is irradiated to a photosensor (an SOS sensor).
Conventionally, such a photosensor is housed in a package independent of a laser diode and dealt as a discretely provided component element. Practically, the driving circuit board for the laser diode and the signal detecting circuit board for the photosensor are connected to each other via a wiring harness. Nevertheless, this conventional structure raises critical problem by causing unwanted noise to easily infiltrate into the harness from external sources.
On the other hand, a laser beam scanning optical system disclosed in the U.S. Pat. No. 4,888,647 is so structured that the laser beam reflected by a polygon mirror returns in the direction of the laser diode. This optical system arranges a photo-volume monitoring photodiode provided on the back of the laser diode to simultaneously function as an SOS sensor. Nevertheless, it is difficult for this laser beam scanning optical system to fully focus the laser beam reflected by the polygon mirror on the photo-volume monitoring photo-diode. Furthermore, it is not always a desirable practice to have the laser beam irradiate the laser diode.